This invention relates to a high density pulp screening apparatus provided with first and second chambers in a papermaking machine or a waste paper processing machine, for example, and more particularly, to a high density pulp screening apparatus for facilitating dissociation and removal of foreign matters or substances from pulp by causing violent turbulence in the first chamber to effect refining operation of the high density pulp.
There are number of pulp screening apparatus generally known in a prior art adapted specifically to refine and screen the high density pulp in form of slurry or suspension, called hereinbelow pulp slurry. One typical prior art pulp screening apparatus is described hereunder in conjunction with FIG. 1, in which a pulp screening apparatus for use in a waste paper processing machine, for example, comprises a casing of generally cylindrical configuration. Within the casing is disposed a screen plate 20 vertically as viewed in FIG. 1 so as to define first and second chambers 21 and 22 on both sides of the screen plate 20 in the casing, and an impeller 23 provided with two blades extending in opposite directions is disposed adjacent to the screen plate 20 in the first chamber 21. The impeller 23 is rotated in the first chamber 22 by a drive mechanism, generally an electric motor 18 connected through a shaft 14 extending across the second chamber 22 to stir pulp slurry inside the first chamber 21.
An intake port 2 for introducing the pulp slurry is provided on the wall of the first chamber 21 directly facing the central portion of the impeller 23 and a discharge port 3 is also provided on the circumferential wall thereof for removing heavy foreign matters therethrough separated from the pulp slurry.
In the second chamber 22 a delivery port 7 is provided on the wall thereof for transferring the refined pulp slurry as an accepted stock.
The impeller used herein comprises a round boss 25 in the central portion and a pair of stirring blades which are extending from the boss into a diameterically opposed directions. The width of the blade generally diminishes from one end adjacent to the boss 25 to the other end farther from the boss. The above-described prior type pulp screening apparatus operates in the following manner.
When the impeller 23 is driven and rotated by the motor 18, the pulp slurry is agitated in the first chamber 21. As the blades of the impeller 23 move forward, the generally flat front face thereof propels the pulp slurry forward and forces the same into the screen plate 20.
The screen plate, in the meantime, is provided with a number of longitudinal slit openings 10 in a properly spaced and parallel relationship to one another for filtering the pulp slurry.
In the forward area of the impeller blade the pulp slurry is shoved and forced into the slit openings 10. On the contrary, in the rearward area of the blade, i.e. in the rear of the flat front face, negative pressure is generated and thus a portion of the pulp slurry which had entered into the second chamber 22 is dragged back into the first chamber. This reverse flow helps to keep the slit openings 10 from being clogged and permits continuous passing of the pulp slurry for screening operation.
The pulp slurry which has been transferred from the first chamber 21 into the second chamber 22 through the screen plate 20 is judged to be accepted and is then discharged from the delivery port 7 of the second chamber for further processing.
Meanwhile, the foreign matters and particles which have failed to pass the screen plate 20 are drained out from the discharge port 3.
In order to assure the continuous screening operation the pulp slurry must be uniformly agitated by means of the impeller 23.
However, the effect of agitation becomes weak as the distance from the impeller becomes larger, and this tendency becomes stronger with the density of the pulp slurry which is supplied. This effect of agitation is manifest at the farthest portion of the impeller and causes the slurry flow to be stagnant, which may adversely lead to a dehydrated condition of the pulp and eventually may result in clogging of the slit openings 10 of the screen plate 20. To prevent such clogging phenomenon, the impeller 23 must be rotated at a high velocity of about 30 m/sec. at the periphery, for example and yet, the maximum density of the pulp slurry within the disposable limit was about 3 percent.
Also with respect to the prior art pulp screen apparatus, with a single impeller rotating in one direction, the first chamber becomes a forced vortex chamber. This causes the pulp slurry to flow and strike the screen surface from an oblique direction. The oblique entry of the pulp slurry degrades the efficiency in the screening performance.
Furthermore, the first chamber has such a pressure distribution that the pressure becomes lower towards the central portion thereof and becomes higher towards the peripheral portion thereof, with the accelerated pressure increase from the center towards the outer periphery as shown in FIG. 2. Thus, the screening capacity of the screening plate is greater towards the outer portion and becomes reduced at the central portion thereof.